Fiberoptic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
The drive towards greater bandwidth has led to use of precision, wavelength-specific DWDM devices that require careful adjustment in order to provide a transmission output at the narrowly separated channel spacings. As tunable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuation become increasingly important. In particular, tunable telecommunication transmitter lasers are susceptible to non-optimal positioning of tunable elements due to environmental thermal fluctuation that results in wavelength instability and reduced transmitter output power. There is currently a need for a telecommunication transmitter laser which provides for simple and accurate adjustment of tunable elements to reduce losses and wavelength stability associated with thermal fluctuation and other environmental factors present during laser operation.
The invention is a laser apparatus and method that uses active thermal adjustment of a laser cavity reflective element to minimize losses and provide wavelength stability. The apparatus of the invention, in general terms, is a laser comprising first and second reflectors defining a laser cavity, and a compensating member coupled to at least one of the reflectors and configured to thermally position the one reflector with respect to the other reflector. The compensating member may be coupled directly to the first reflector and configured to position the first reflector with respect to the second reflector. The thermal positioning may be carried out by a thermoelectric controller operatively coupled to the compensating member and configured to thermally adjust the compensating member by heating or cooling thereof.
More specifically, the laser apparatus may comprise a gain medium having first and second output facets and emitting a light beam from the first output facet along an optical path. The first reflector is positioned in the optical path, with the second output facet and first reflector defining a laser cavity. The compensating member may be thermally conductive and have a high coefficient of thermal expansion.
In certain embodiments the gain medium and first reflector may be passively athermalized or thermally stabilized with respect to each other. In this regard, the laser may further comprise a base, with the compensating member and gain medium mounted on the base. The base has a first, selected coefficient of thermal expansion, and the compensating member has a second, selected coefficient of thermal expansion, and the base and compensating member are dimensioned and configured to passively athermalize the external cavity. The passive thermal stabilization may be carried out concurrently with the active thermal control of the end mirror by heating or cooling of the compensating member.
The laser apparatus may further comprise a grid generator positioned in the optical path before the end mirror, and in some embodiments may further comprise a channel selector positioned in the optical path before the end mirror and configured to tune or adjust the output wavelength of the laser. The grid generator may comprise a grid etalon mounted to the base. The channel selector may be coupled to the base via a drive or tuning assembly. The grid etalon and channel selector may also be subject to passive thermal stabilization through the base. The grid etalon may additionally be coupled to a thermoelectric controller and subject to active thermal control.
In other embodiments the laser may also comprise a detector associated with the external cavity that is configured to detect losses aspects of the external cavity. The detector may be an optical detector positioned to monitor optical output from the external cavity, or may be a voltage sensor positioned to monitor voltage across the gain medium. Error signals derived from the output of the detector may be utilized by a controller to adjust the external cavity by thermal positioning of the end mirror via heating or cooling the compensating member.
The laser may further comprise a dither element operatively coupled to the external cavity and configured to introduce a detectable frequency modulation into the external cavity. The dither element may be associated with the end mirror or located elsewhere in the external cavity. The frequency modulation introduced by the dither element results in a known or predictable intensity and/or phase variation in optical feedback from the external cavity to the gain medium. This intensity and/or phase variation from the dither is detectable in either the monitored voltage across the gain medium or the optical output of from the external cavity. The positioning of the end mirror via heating or cooling the compensating element effects the phase and intensity of the modulation signal, and the magnitude and phase of the modulation signal as detected via voltage or optical power modulation may be used to generate an error signal. The error signal is usable to position or otherwise adjust the end mirror to null the error signal according to thermal positioning of the end mirror by the compensating member.
The method of the invention is a method of laser operation that comprises, in general terms, providing first and second reflectors that define a laser cavity, and adjusting the laser cavity by thermally adjusting a compensating member coupled to at least one of the reflectors. The thermally adjusting of the compensating member comprises heating or cooling the compensating member with a thermoelectric controller coupled to the compensating member. The method may further comprise passively athermalizing or thermally stabilizing the laser cavity, and monitoring external losses associated with the laser cavity. The thermal adjusting may be carried out according to error signals derived from the monitoring of losses associated with the external cavity. The method may further comprise introducing a frequency modulation into the external cavity, and deriving error signals according to amplitude and phase of detected frequency modulation.